Battery safety is an industry priority. A single cell failure may be an ignition source for other cells in the battery pack, and/or surroundings. Preventing or eliminating failure mechanisms in lithium-ion battery design is part of maintaining safety in battery-operated systems.
Standard lithium-ion cell manufacturing processes create an exposed aluminum edge where the cathode master coating roll, sometimes referred to as a jumbo roll, is slit or punched to the dimension of the cell electrode. A cell is assembled with layers of cathode, separator, and anode. In some configurations the layers are wound into rolls, and in other they are stacked. Battery design requires the anode to be larger than the cathode. This creates a pocket in the electrode winding where a particle or contamination can land in, or migrate into.
It is generally not possible to completely avoid contaminants in the process of manufacturing battery cells. Ambient contaminants cannot be completely avoided, and the cell manufacturing process itself often introduces contaminants from machinery wear and tear, etc. Accordingly, lithium-ion battery cells typically include some contaminants. These contaminants are typically in the form of very small particles. The contaminating particles often fall in the pockets between the anode and cathode.
In some cases, the particle may not cause an immediate failure and the cell can pass all testing before leaving the factory. However, the particle may cause a latent defect. For example, when a cell is charged and discharged, the electrodes may swell and compress the pocket. This can cause the particle to puncture a separator which normally separates the anode from the cathode. When the separator is punctured, the particle can create an internal short circuit from the anode active material to the cathode active material, or the anode active material to the cathode current collector. A short between the cathode current collector (aluminum) and anode active material can often be a critical short. A critical short may cause a partial or total failure of the cell, and in some cases may cause an explosion or other undesirable effects.